PROJECT SUMMARY Attention-Deficit Hyperactivity Disorder (ADHD) affects approximately 7% of children, making it one of the most common neurodevelopmental disorders worldwide. ADHD presents a significant public health burden because the disorder causes hyperactivity, impulsivity, and inattention symptoms that affect individuals in social, academic, and occupational settings, and can affect individuals well into adulthood. To date, no singular pathogenic cause of ADHD has been found and ADHD etiology is most likely multifactorial. While most patients with ADHD are treated with the stimulant methylphenidate, long-term use is associated with cognitive side effects and substance abuse, and significantly decreased quality of life still occurs. Elucidating the etiology and pathogenesis of ADHD is therefore imperative. Our group recently showed that exposure to pyrethroid pesticides induces alterations in dopaminergic proteins and ADHD-like behaviors in a mouse model. Interestingly, pyrethroid pesticide exposure is higher in low socioeconomic status populations, and these populations are also at increased risk of ADHD. Low socioeconomic status populations also experience greater amounts of chronic psychosocial stress, which alters the dopaminergic system in a rodent model. Thus, certain populations of children may be exposed to both pyrethroid pesticides and chronic psychosocial stress, two environmental exposures that are known to change components of the dopamine system. We thus hypothesize that the combined dopaminergic effects of pyrethroid pesticide and stress exposure will lead to additive effects on the dopamine system, ultimately increasing the risk of developing ADHD. We will test this hypothesis through the following aims. Aim 1 is to determine the neurobehavioral and neuropathologic effects of combined neurodevelopmental pyrethroid and corticosterone exposure in mice. More specifically, we will assess behavioral and neurochemical changes that occur in the dopamine system in response to pyrethroid pesticide and stress hormone exposure alone and in combination from gestation through adulthood. To do so, we will utilize a neurodevelopmental mouse model that we previously validated for examination of dopamine-related changes after pyrethroid pesticide exposure. Aim 2 is to determine whether key dopaminergic regions exhibit altered levels of DNA methylation in response to combined neurodevelopmental pyrethroid and corticosterone exposure in mice. To study these epigenetic effects, we will utilize the neurodevelopmental mouse model described in Aim 1, isolate midbrain tissue, and use targeted next-generation sequencing to assess whether DNA methylation changes at key dopaminergic gene regions. Altogether, this research will yield important mechanistic insight into the consequences of pyrethroid and stress hormone exposures in the dopaminergic system, in the context of neurodevelopment and ADHD. Understanding mechanisms of ADHD etiology and pathogenesis will provide valuable information for development of more effective ADHD pharmacotherapies and behavioral interventions, as well as the implementation of diagnostic screening targeted towards at-risk populations.